The present invention is generally related to suspension systems for wheeled vehicles. More particularly, the present invention is related to suspension systems for wheelchairs.
Motorized front and mid-wheel drive wheelchairs having rear casters generally do not include a resilient suspension for those casters. These wheelchairs rely only on the compliance of inflatable tires on the casters to cushion the rider and wheelchair components when the wheelchair is driven across rough or uneven terrain and to absorb dynamic loads caused by accelerating and decelerating the wheelchair. Although this conventional approach is satisfactory under some conditions, it does not provide an acceptable ride for the range of conditions encountered during normal use. A smooth ride over a broad range of conditions would reduce fatigue and the possibility of injury to the rider and damage to the components of the wheelchair, such as the electrical and drive systems.
Some rear-caster wheelchairs do have resilient suspensions for the rear casters. However, these suspensions deflect under load regardless of the swivel orientation of the caster""s wheel. A problem with these suspensions is that they can make the wheelchair unstable when driven backwards down a slope, off a curb or the like.
When a wheelchair having rear casters is driven backwards, the caster wheels swivel and become positioned toward the front of the wheelchair. In this position, the wheelbase is shortened and the casters are located closer to the center of gravity of the wheelchair and rider, increasing the proportion of the weight of the wheelchair and rider supported by the casters. As the caster suspension deflects under the increased load, the wheelchair tilts, moving the center of gravity even closer to the rear casters, decreasing the stability of the wheelchair. When driven backward down a slope, off a curb or the like, the load shifts even more to the rear casters and causes the suspension to deflect further, causing the wheelchair to tilt at an even greater angle and further decreased stability. On severe down-slopes, the tilt angle of the wheelchair may become so large that the center of gravity falls outside the wheelbase, causing the wheelchair to tip over and injury to the rider.
Another problem with a wheelchair having rear casters is that the casters tend to swivel sideways when ascending a curb or other upstanding obstacle. When a front or mid-wheel drive wheelchair ascends a curb, the drive wheels are first driven up the curb. When the drive wheels are on top of the curb but the casters are not, the wheelchair is in a relatively severe rearward-tilt position. In this position, the swivel axis of each caster is tilted rearward with respect to the vertical, making the caster unstable. In this rearward tilt position, the weight supported by the caster tends to cause the caster to swivel from its unstable, trailing position until it is at least sideways with respect to the face of the curb. As the wheelchair moves forward, the caster is dragged sideways up the face of the curb, causing damage to the caster. This effect is even more of a problem when the wheelchair is driven up a curb at an angle of other than 90xc2x0 with respect to the curb.
One solution to the tendency of the caster to approach a curb sideways when the wheelchair is ascending the curb is to mount the casters to the wheelchair with their swivel axes tilted toward the front of the wheelchair rather than vertical. The forward tilt angle counteracts the rearward tilt of the swivel axis caused by the tilt of the wheelchair as it is ascending a curb. This solution is described in British Patent Application no. 2 199 291 A. However, as discussed in that application, tilting the swivel axis too far forward causes the caster end of the wheelchair to raise and lower each time the casters swivel from a trailing position to a leading position and vice versa. This motion can cause great discomfort to the user.
A further problem with a wheelchair having conventional casters is that the casters are prone to xe2x80x9cflutter,xe2x80x9d i.e., sideways oscillations about their swivel axes. Flutter is particularly a problem when the wheelchair is driven relatively fast in a straight line and the casters engage bumps or other irregularities in surface being traversed. The irregularities can cause the casters to swivel sideways, causing the casters to oscillate side-to-side, or flutter. The tendency of the casters to flutter can be reduced by mounting the casters to the wheelchair so that their swivel axes are tilted forward. However, too little a tilt angle will not eliminate flutter, and too great a tilt angle will cause the caster end of the wheelchair to raise and lower as the casters swivel from a trailing position to a leading position and vice versa.
The present invention is directed to a caster and suspension assembly. In one aspect of the invention, the assembly includes a rocker arm having a pivot axis. The rocker arm is pivotally attached to the rocker arm support such that the rocker arm may be pivoted relative to the rocker arm support about the pivot axis. A caster is swivelably attached to the rocker arm. A resilient member has a pair of spaced-apart ends. One end of the resilient member engages the rocker arm support, and the other end engages the rocker arm. A rotation limiter engages the rocker arm support and limits the rotation of the rocker arm relative to the rocker arm support in one direction. The assembly has an unloaded configuration wherein the resilient member biases the rocker arm into engagement with the rotation limiter.